fix clobbering app updates during staging

There was a race between calling app.refresh and checking if the task is
current, and updating the detected buildpack attribute. It called .save, which
meant any changes that happend between there would get blown away.

This replaces .save usage with .update, which only updates the given
attributes.

[finishes #58774740]
Merge pull request #103 from cloudfoundry/fix-cc-staging-clobbering

fix clobbering app updates during staging